It is known to provide a micro-fluidic bio-sensor, comprising an electrically conductive layer and an overlaid insulating material. Such devices are used for testing, typically liquid, samples enzymatically, as a cheap and simple alternative to laborious laboratory tests. WO 2005/007866 discloses one such device for the measurement of ion transport, having an upper chamber piece comprising at least one well, with a chip attached to the bottom of the upper chamber piece, the wells aligning with at least one ion transport measuring means.
The electrically conductive layer of such devices generally comprises a plurality of electrical contact pads having the relevant enzyme attached to the surface thereof, and liquid receptacles are formed over the top of these contact pads, into which the solution to be tested is poured.
The electrochemistry of the reactions between the surface enzymes and the liquid to be characterized can then be measured using cyclic voltammetry (hereinafter referred to as ‘CV’), which allows for the determination of relevant characteristics relating to the liquid. For optimum CV to occur, the liquid receptacles associated with different contact pads must be of a consistent volume, which is only achievable by being able to form regular, accurately sized liquid receptacles or ‘wells’.
There are many plausible ways of creating such wells based on existing technology, each of which have particular drawbacks. The first such technique is that of the photo-imaged solder mask. This involves the application of a thin, lacquer-like polymer to the surface of a printed circuit, and this is a technique commonly used in the formation of printed circuit boards (hereinafter referred to as ‘PCB’).
However, for optimum CV, there must is a minimum required depth, typically of 100 μm, in order to contain sufficient fluid in each well. This is very expensive for solder masking, since not only is the raw material expensive, but the application process also has many involved steps.
Additionally, where a deep solder mask has been utilized, there will be a tendency for the sidewalls of the defined recess or well to be sloped rather than straight, which limits the effectiveness of the wells leading to inconsistencies in the well volume between different wells and consequently test results.
An alternative method of well creation is to use a thermally laminated cover layer. This involves the application of a thermally-set laminate adhesive over the top of the printed circuit. However, in this scenario, the wells must be cut, punched or ablated from the laminate, which is less than ideal. Again this results in an expensive, multi-step process.
The likelihood of adhesive flow must also be considered; the thermally-set adhesive is prone to flowing like a viscous fluid during the setting process, which again can result in inconsistently sized and shaped wells, reducing the efficacy of the CV analysis.
A final option would be to use a pressure sensitive adhesive instead of a thermally-setting equivalent. However, the extraction of the wells is a problem, since the nature of the bonding of this type of adhesive increases the probability of ‘tear-out’, that is, the formation of undesirable voids in the insulative layer during punching or drilling of the wells.
Additionally, since such adhesives are generally quite ‘spongy’ or porous, they are very susceptible to moisture absorption, which may have undesirable effects on the enzymes of the circuitry. To counteract this, pressure-sensitive adhesives are preferably created with very shallow wells. However, shallow wells increase the likelihood of enzymatic cross-contamination between wells, which is undesirable.
Research has been performed using the above options, and it is thus an object of the present invention to provide a bio-sensor which avoids or obviates the stated problems.